1. Field of the Invention
The present invention relates to signal processing, and more specifically, to multi-carrier modulation techniques, such as orthogonal frequency division multiplexing (OFDM), used in signal transmission and reception.
2. Description of the Related Art
Multi-carrier modulation techniques, such as orthogonal frequency division multiplexing (OFDM), are used in wired and wireless communication systems such as local area networks, fixed and mobile metropolitan area networks, and cellular phone systems. In general, multi-carrier modulated symbols are generated by dividing a frequency spectrum into smaller frequency subcarriers (a.k.a., tones) and modulating the subcarriers with parts of one or more data signals. The one or more data signals may be acquired from one or more sources (e.g., users), and each multi-carrier modulated symbol may transmit data from more than one source.
FIG. 1 shows a simplified block diagram of one implementation of a prior-art OFDM transmitter 100. Transmitter 100 has data symbol mapper 102, which receives a serial bitstream of digital data from upstream processing. The serial bitstream is divided into groups of bits, and each group is mapped into one or more data symbols to generate a serial stream of data symbols. Mapping may be performed using one or more suitable techniques such as quadrature phase-shift-keying (QPSK) and quadrature amplitude modulation (QAM).
Serial-to-parallel (S/P) converter 104 converts the serial stream of data symbols received from data symbol mapper 102 into D parallel streams of data symbols. Subcarrier mapper 106 assigns the D parallel data symbol streams to N subcarrier frequencies (i.e., tones), where the N subcarrier frequencies are arranged orthogonally to one another. In particular, each parallel data symbol stream is assigned to a separate output of subcarrier mapper 106, where each output corresponds to a different one of the N subcarriers. Note that, for ease of discussion, this implementation assumes that the number D of data symbol streams is equal to the number N of subcarriers. According to other implementations, a number D of data symbols and a number P of pilot symbols may be assigned to the N subcarriers, where there may be a number U of unused (i.e., free) subcarriers, such that N=D+P+U. The N outputs (e.g., Z=Z1, . . . , ZN) of subcarrier mapper 106 are then provided to inverse fast Fourier transform (IFFT) processor 108. IFFT processor 108 transforms each set of N outputs from subcarrier mapper 106, where each output in a set corresponds to a different one of the D data symbols, into one OFDM symbol, comprising N time-domain complex numbers (e.g., z=z1, . . . , zN).
Each OFDM symbol is then prepared for transmission. First, a cyclic prefix, comprising C complex numbers, is inserted onto each OFDM symbol by cyclic-prefix inserter (CPI) 110. This prefix enables the receiver to cope with signal echoes that result from multi-path reflections. Next, each set of N time-domain complex numbers and each corresponding set of C cyclic prefix complex numbers are converted from parallel to serial format by parallel-to-serial (P/S) converter 112. The output of P/S converter 112 may be further processed using digital-to-analog conversion, radio-frequency modulation, amplification, or other processing suitable for preparing the OFDM symbols for transmission.
During IFFT processing, the data symbols are applied to modulate the corresponding subcarriers, and the modulated subcarriers are added together, often constructively, creating an OFDM symbol with a number of high and low amplitude peaks. Due to the varying nature of the transmitted data, the height of these peaks will typically vary within each OFDM symbol and from one OFDM symbol to the next. Further, some of these peaks can become relatively large compared to the average amplitude level of the OFDM symbol, resulting in a relatively large peak-to-average power ratio (PAPR). The PAPR for an OFDM symbol may be represented as shown in Equation (1) below:
                    PAPR        =                                            max                              n                =                1                            N                        ⁢                          (                                                                                      z                    n                                                                    2                            )                                                          1              N                        ⁢                                          ∑                                  n                  =                  1                                N                            ⁢                                                                                      z                    n                                                                    2                                                                        (        1        )            
In Equation (1), zn is the nth sample of the OFDM symbol z, and the max function in the numerator determines the largest value of |zn|2 for n=1, . . . , N.
An OFDM symbol having a relatively large PAPR may become distorted during power amplification. One or more relatively large samples of the OFDM symbol may attempt to drive the output of the amplifier towards its maximum output level. Prior to reaching the maximum output level, the input-to-output relationship of the amplifier may become non-linear resulting in non-linear distortion of the OFDM symbol. When the amplifier's maximum output level is reached, the amplifier clips the sample, resulting in further non-linear distortion of the output signal. Non-linear distortion affects the quality of the signal, and consequently, the receiver may experience difficulties in recovering the transmitted data.
A number of different methods have been employed to reduce the effects of non-linear distortion by the amplifier or eliminate non-linear distortion altogether. In one such method, the transmitter employs a larger amplifier capable of outputting higher power levels. Typically, the larger amplifier is operated with considerable back-off (i.e., the amplifier can be operated at a lower average power) to ensure that the amplifier remains in its linear region of operation even during peak signal events. However, using a larger amplifier in such a manner is inefficient.
In another such method, the transmitter performs amplification in stages to achieve the desired output level. In this method, each stage comprises an amplification step and a filtering step. The amplification step results in relatively minor clipping of the larger samples of each OFDM symbol. The filtering step smoothes out each OFDM symbol to reduce the amount of distortion that occurred in the preceding amplification step. This successive clipping and filtering process is repeated until the desired amplification level is achieved. By amplifying a signal in this manner, the amount of distortion can be reduced over that of an equivalent single-stage amplifier.
In yet another such method, numerous pseudo-random scrambling sequences are applied to the OFDM signal in the frequency-domain (e.g., the output subcarrier mapper 106), and the scrambling sequence that results in the lowest PAPR after IFFT processing is selected. Since the scrambling sequence selected is not known by the receiver, the scrambling sequence may be transmitted to the receiver on another channel, or the sequence may be detected ‘blindly’ by the receiver. In the later case, the receiver tests all possible sequences and picks the most likely sequence.
Yet further methods, known as tone reservation (TR) methods, attempt to reduce the PAPR for each OFDM symbol. In such methods, a number of frequency subcarriers (i.e., tones) are reserved for transmitting non-data symbols that have the express purpose of reducing PAPRs of OFDM symbols.
FIG. 2 shows a simplified block diagram of one embodiment of a prior-art transmitter 200 which uses a TR approach for reducing PAPR. Transmitter 200 has data symbol mapper 202 and S/P converter 204, which perform operations analogous to those of the equivalent elements of transmitter 100 to generate sets of D parallel data symbols. Subcarrier mapper 206 assigns each set of D data symbols to a set of N subcarriers such that M subcarriers are not assigned a data symbol. The M subcarriers are reserved a priori for transmitting PAPR-reduction symbols. Note that, in certain embodiments, each set of N subcarriers may be assigned D data symbols and P pilot symbols, where there are a number M of reserved subcarriers and a number U of unused (i.e., free) subcarriers, such that N=D+M+U+P. Each set of N outputs (e.g., Z=Z1, . . . , ZN) from subcarrier mapper 206 is provided to IFFT processor 208, which performs operations analogous to those of IFFT processor 108 to transform each set into an OFDM symbol z, comprising N time-domain complex numbers (e.g., z=z1, . . . , zN).
PAPR-reduction symbol generator 210 receives each OFDM symbol z and compares the PAPR of each symbol to a specified PAPR threshold value, which represents an acceptable level of PAPR reduction for the OFDM symbol. If the PAPR of an OFDM symbol z is less than the PAPR threshold value, then the OFDM symbol z is output from PAPR-reduction symbol generator 210 as OFDM symbol {circumflex over (z)} (i.e., z={circumflex over (z)}). If the PAPR of an OFDM symbol exceeds the PAPR threshold value, then PAPR-reduction symbol generator 210 generates a set of M PAPR-reduction symbols using any one of a number of approaches (as discussed in further detail below) and provides the set to subcarrier mapper 206. Note that, in other implementations, PAPR-reduction symbol generator 210 may always generate a set of M PAPR-reduction symbols for each OFDM symbol. In such implementations, the comparison between the PAPR of an OFDM symbol z and a specified PAPR threshold value may be omitted.
Subcarrier mapper 206 assigns the set of M PAPR-reduction symbols to the M PAPR-reduction subcarriers and outputs N complex numbers (e.g., Z=Z1, . . . , ZN), which includes the M PAPR-reduction symbols and the D data symbols. The N complex numbers are then transformed by IFFT processor 208 to generate a PAPR-reduced version of OFDM symbol z, which is provided to PAPR-reduction symbol generator 210. This process is repeated until the PAPR of the PAPR-reduced OFDM symbol z is less than the PAPR threshold value, and, once this condition occurs, the PAPR-reduced OFDM symbol z is output from PAPR-reduction symbol generator 210 as OFDM symbol {circumflex over (z)} (i.e., z={circumflex over (z)}). Each OFDM symbol {circumflex over (z)} is then prepared for transmission using cyclic-prefix inserter 212 and P/S converter 214, which perform operations analogous to those of the equivalent elements of transmitter 100, and any other processing suitable for preparing OFDM symbols for transmission.
By assigning PAPR-reduction symbols to reserved tones, the peak values of time-domain OFDM symbols may be reduced without affecting the individual data symbols. The designer of an OFDM transmitter has a large degree of freedom to select PAPR-reduction symbols to assign to reserved tones, and numerous methods have been employed for selecting symbols that will sufficiently reduce PAPRs. One such approach for selecting PAPR-reduction symbols involves performing iterative combinatorial searches. As an example of a combinatorial approach, suppose a transmitter modulates data using 16-quadrature amplitude modulation (16-QAM) and reserves 8 tones for PAPR-reduction symbols. The transmitter will consider 168 different combinations of PAPR-reduction symbols to place on the reserved tones of each OFDM symbol, and will select the combination of symbols that generates the lowest PAPR.